Post-menopausal osteoporosis, a global public health problem, has for decades been attributed solely to declining estrogen levels, and although follicle stimulating hormone (FSH) levels rise sharply in parallel, a direct effect of FSH on the skeleton has never been explored. The only ascribed function of FSH is ovarian estrogen secretion. We speculate that, in addition to declining estrogen, FSH drives the decreases in bone mass during the early menopause by stimulating the osteoclast, the cell that resorbs bone. Mice devoid of FSH or its receptor do not display hyper-resorption or bone loss despite being severely hypogonadal. That FSH is pro-resorptive is supported by in vitro evidence for a G-protein coupled FSH receptor (FSHR) on the osteoclast. FSH also enhances the release of the osteoclastogenic cytokine TNFa from osteoclast precursors, and additionally, promotes osteoclast survival. The hypothesis emerging from our study, supported by the tight correlations between bone mass and serum FSH in humans, is that circulating FSH directly stimulates osteoclastic bone resorption. We will therefore investigate in Specific Aim 1 whether FSH causes bone loss in vivo independently of lowered estrogen. For this, we will administer or over-express FSH in mice lacking the two estrogen receptors, ER??-/-, as well as GnRH-deficient hpg mice. We will also examine whether selective FSHR deletion in the osteoclast will prevent ovariectomy-induced bone loss, and whether transgenic reconstitution of the FSHR in FSHR-/- osteoclasts will restore resorptive activity. In Specific Aim 2 we determine the mechanism of the FSH effect. We will first study the mechanism of FSH-induced TNFa expression, and then, using TNFa-/- mice, elucidate if the entire effect of FSH is TNFa-dependent. Finally, using Akt-deficient cells, we will also determine whether the pro-survival action of FSH is Akt-mediated. If FSH is proven to be pro-resorptive in vivo, we envisage attenuating FSH in humans to a skeletal advantage without compromising ovarian function, for example by a monoclonal antibody. The latter premise arises from our observation that FSH haploinsufficiency in mice increases bone mass, while sparing the ovaries. The significance of this work thus lies not only in our challenging an archetypal paradigm, estrogen deficiency, as being the full explanation for menopausal bone loss, but also in establishing that pituitary hormones, such as FSH, act beyond traditional target endocrine organs.